The default value for a currency field should be "empty" and not "€ 0,00"

Use case: I have a database of newspapers and one of the field is the price of each newspaper. Some newspapers have a price, some newspapers are free and there are some newspapers for which I currently don’t know the price. The problem is that when the price field is formated as “Currency”, the default value is “€ 0,00”. So, it’s not possible for me to know which newspapers are actually free and which are the newspapers for which I currently don’t know the price.
In order to bypass this issue, I’ve configured the price field as a “text” field, but this is not the ideal solution, as this doesn’t allow to do mathematical calculations on this field.
If the user didn’t inserted any information on a specific field, that field should not be filled with € 0,00 (or with anything else), as it can result in wrong information (as it happens on this example).
So, can you please change the default value of a currency field from “€ 0,00” to “empty”?